Subnets

#networking #subnets #network

In this blog, I’ll break down subnetting, show you how it works, and walk you through my subnetting project (complete with diagrams and code).

Let’s untangle the magic behind those /24, /16, and 255.255.255.0 masks. 🎭

🚦 Why Subnetting Matters

"Without subnetting, your network’s like a city with no street names — total chaos."

Imagine 10,000 devices trying to talk on the same street with no addresses. That’s what a flat IP network looks like.

Subnetting introduces:

📦 Structure
🚀 Routing efficiency
🔐 Security boundaries

Let’s break it down 👇

🌐 What is a Subnet?

A subnet (short for subnetwork) is a smaller network within a larger one, created by dividing an IP range using a subnet mask.

For example:

🧮 IP Address: 135.70.1.0

🎭 Subnet Mask: 255.255.255.0

We’re saying:

First 3 octets = Network
Last octet = Hosts

This helps:

Routers 🚚 know where to send packets
Admins 🧑‍💻 manage traffic flow between devices and regions

🧩 How Subnetting Works

A subnet mask splits the IP address into:

🧱 Network bits — define the subnet
🏠 Host bits — define the actual devices

Think of each subnet like a neighborhood in a city:

Devices (houses) are easier to locate, control, and communicate within that area 🏡

📸 Visual Aid:

🔢 IPv4 Classes and Subnet Masks

Here’s a handy chart that shows the basics of IPv4 classes:

🏷️ Class	🛡️ Subnet Mask	🔁 # of Networks	👥 Hosts per Network
A	`255.0.0.0`	`128 networks`	`16,777,216 hosts`
B	`255.255.0.0`	`16,384 networks`	`65,536 hosts`
C	`255.255.255.0`	`2,097,152 networks`	`256 hosts`

🔍 These classes define how many networks and hosts you can support. For example, Class C is often used in homes and small offices!

🛠️ How I Built My Own Subnet Planner in Go

Ever wanted to divide networks smartly for homes, offices, or cafés? 🍵🏢🏡

I created a Subnet Calculator in Go that:

✅ Accepts an IP address and the desired number of subnets
🧮 Calculates the new subnet mask
✂️ Splits the network accordingly
🔍 Shows usable ranges for each area

⚙️ Go Code: Subnet Planner

package main

import (
    "net"
    "fmt"
)

func main() {
    _, ipNet, err := net.ParseCIDR("192.168.0.0/16")
    baseIP := ipNet.IP
    oldPrefix, _ := ipNet.Mask.Size()

    if err != nil {
        panic(err)
    }

    newPrefix := 24
    numSubsets := 1 << (newPrefix - oldPrefix)

    var subnets []net.IPNet

    for i := 0; i < numSubsets; i++ {
        subnetIP := make(net.IP, len(baseIP))
        copy(subnetIP, baseIP)

        subnetIP[2] = byte(i)
        subnet := net.IPNet{
            IP:   subnetIP,
            Mask: net.CIDRMask(newPrefix, 32),
        }
        subnets = append(subnets, subnet)
    }

    areas := []string{"🏠 home", "🏢 office", "☕ cafe", "🌳 park", "📚 library"}

    for i, area := range areas {
        if i < len(subnets) {
            fmt.Printf("%-10s : %s\n", area, subnets[i].String())
        }
    }
}